Sorry, but this is the first time attaching screenshots.
I have no idea what the error detail is telling me.
I tried to set the label from both solydx8-1 and solydx8-2.
I am assuming that there is a bug in GParted.

But both labels solydx8-1 and solydx8-2 are already set? And the error message "Set partition label to "solydx9"" doesn't fit with the picture which has "Solydx9-1"? Did you try several names?

Probably not:
1. Label characters - I always use the underscore _ but I don't remember why
2. There was a bug in ext3 which resulted in that error message but that should not be a problem anymore.
So that leaves:
3. Your filesystem has a serious problem which is unlikely if everything else works.
4. Kernel and ext4 tools are not compatible - unlikely on a fresh install.
5. Which system were you running? The one on the partition you wanted to rename? (Which could be protected in gparted by setting it to read-only.)

So, in short, I don't know. To spare the next visitor reading the output you posted, the relevant error message is:

I tried to set sda8 from 2 earlier versions of Solydx on sda3 and sda7. They both had GParted 0.19.0. The new install of Stretch uses GParted 0.25.0, but of course you can only change unlocked partitions. Is there a way that I can get GParted 0.25.0 on Jessie?

This has nothing to do with gparted. gparted is just a gui, in this case for e2label, which is part of the e2fsprogs package. This package has to be correspondent to the kernel version used, so you should not try to update anything separately.
You can boot the SolydX9 live image, start gparted and see how things work from there.

Solved:
Only GParted invoked from a partition which has the same username as another, target, partition can change the partition label.
If the two partitions don't use the same username then the actor will not have write permissions on the target.
I installed a second partition with solydx9 and set it up with the same username. Then the second can set the name of the first and vice versa. At least that solved my problem.